The amount of radioactive carbon-14 in the atmosphere rose dramatically in 773 AD. Astronomers think the culprit must have come from space but more than that, they can’t say

In November 2012, a group of Japanese scientists made a remarkable announcement that much of the world’s media missed.

Fusa Miyake and pals at Nagoya University in Japan were studying tree ring records which can be resolved almost to the exact year. These guys found that the concentration of carbon-14 in Japanese cedar trees suddenly rose between 774 AD and 775 AD. Others have since found similar evidence and narrowed the date to 773 AD.

So what caused this sudden increase in carbon-14? Astronomers agree that the culprit must have come from space, beyond that they are at loggerheads. Now the battle is hotting up to correctly identify the extraterrestrial source of this medieval carbon spike and resulting scientific spat is becoming increasingly entertaining.

Carbon-14 is continually generated in the atmosphere by cosmic rays hitting nitrogen atoms, causing them to absorb a neutron. But because carbon-14 is radioactive, it naturally decays back into nitrogen with a half-life of about 5700 years.

This constant process of production and decay leaves the amount of carbon-14 in the atmosphere relatively constant. Pick up some carbon today and about one part in a trillion will be carbon-14.

Last month, a group of Chinese scientists pointed out that this process of carbon-14 production doesn’t just occur on Earth but all over the universe, wherever cosmic rays interact with nitrogen. For example, comets often contain ammonia (NH3) ice and so will contain a fraction of carbon-14 produced as they are bombarded by cosmic rays.

That raises an interesting possibility, they said. Perhaps Earth was struck by a comet in AD 773 and the impact showered the planet with carbon-14. Indeed

Today, Ilya Usoskin at the University of Oulu in Finland and Gennady Kovaltsov at the Ioﬀe Physical-Technical Institute in St Petersburg, Russia, firmly rule out this possibility.

They point out that the total amount of extra carbon-14 dumped into the atmosphere must have been equivalent to about 18 kilograms. That allows them to calculate the minimum size of any comet that could have been responsible.

Given the amount of nitrogen that we can see in comets and the percentage of this that could have been converted to cabron-14, Usoskin and Kovaltsov calculate that the culprit could not have been smaller than about 100 kilometres across.

And if so, such an impact would have left significant evidence elsewhere on the planet that would be hard to miss—an impact crater for example. “The absence of an evidence for such a dramatic event makes this hypothesis invalid,” they conclude.

The only other possibility is that the carbon-14 was created in the Earth’s atmosphere itself. There are two ways that might have happened. The first is a nearby supernova that suddenly drenched the Solar System in high energy cosmic rays.

But Miyake and co say there are no historical records of a supernova in 773 AD and neither can astronomers see the remains of any recent supernova that might be to blame.

The second way carbon-14 can be created in the Earth’s atmosphere is if the Sun suddenly belched high energy particles our way. In other words, the Sun might have emitted a superflare 1000 times larger than usual which then engulfed the Earth.

There was a time when astronomers would have immediately ruled out this possibility as well. But last year, astrophysicists calculated that sun-like stars can produce superflares of this size about once every 3000 years.

There are certainly hints in medieval texts that something interesting occurred in the atmosphere at that time. One English text from AD 774 reads: “This year also appeared in the heavens a red crucifix, after sunset ; the Mercians and the men of Kent fought at Otford; and wonderful serpents were seen in the land of the South-Saxons.” And a German text from AD 776 describes two shields burning with red colour and moving above a church during Charlemagne’s campaign against the Saxons.

That’s not really proof of anything but it is a tantalising hint that people noticed something interesting at that time. Clearly, a more detailed study of historical texts from that time is needed. Google Books anyone?

For the moment, however, the cause of the carbon-14 spike remains a mystery. Whatever it was, must have come from space. More than that, who knows.